Vascular thrombosis, particularly in the deep veins of the leg, is a common medical disorder. A thrombosis often creates a great deal of localized pain and discomfort, and if the thrombus becomes dislodged, the resulting embolus may reach the capillary bed of the lung and create a life-threatening situation. Prompt diagnosis and treatment of vascular thrombi can greatly reduce the mortality rate from pulmonary embolism. Several major techniques for the diagnosis of vascular thrombi are known in the art, all of which suffer from various shortcomings.
The International Subcommittee on Diagnostic, Prophylaxis and Treatment of Venous Thrombosis noted in 1979 that "there is a need to develop improved noninvasive methods for the diagnosis of calf vein thrombosis in clinically symptomatic patients" and "current diagnostic tests are not useful for screening asymptomatic patients (other than .sup.125 I-fibrinogen leg scanning) and do not have predictive capacities" (Thrombos. Haemostas., 41, pp. 450-53, 1979).
Plethysmographic and Doppler ultrasound techniques which measures blood volume, blood pressure, or blood flow changes lack sufficient sensitivity to detect partially occluded vessels since substantial occlusion of major vessels is required to produce abnormal results. In addition, the source of the obstruction, i.e., thrombus, inflammation, swollen gland, etc., is not identified.
Contrast media radiography (venography, phlebography or angiography) is probably the most acceptable of the standard techniques available. Drawbacks of this procedure include the pain experienced by the patient and a significant number of other side effects ranging from thrombosis and allergic reactions to death. Many of these side effects have been eliminated with radionuclide venography and angiography although these techniques are expensive and the results are highly dependent on the skill of the individuals performing the techniques.
The above-mentioned techniques merely infer the location and size of a thrombus by the absence of blood flow or a "negative image". A better method would directly detect a thrombus by a "positive image". This has been done using .sup.131 I-fibrinogen which is incorporated biochemically into a growing thrombus producing a "hot" spot or "positive radiographic image". Unfortunately this material is only selective for actively growing thrombi and does not detect aged or nongrowing ones. The latter are the more common type of thrombi which a physician sees in symptomatic patients.
The desirability of a "positive radiographic image" technique has been well recognized. A number of thrombus-specific biomolecules such as urokinase, streptokinase and plasminogen, have been labeled with .sup.99m Tc and tested in animals, but the results have not been reproducible. It is speculated that the variability of these results is caused by difficulties in protein isolation and variability in labeling techniques. Heparin labeled in this manner has been reported to image damaged coronary arteries and myocardium [Padmakar V. Kulkarni et al., "Technetium-Labeled Heparin: Preliminary Report of a New Radiopharmaceutical with Potential for Imaging Damaged Coronary Arteries and Myocardium", J. Nucl. Med., 19, 810-815 (1978) and Padmakar V. Kulkarni et al., "Modified Technetium-99m Heparin for the Imaging of Acute Experimental Myocardial Infarcts", J. Nucl. Med., 21, 117 (1980)]. .sup.99m Tc-heparin is also being investigated for imaging damaged blood vessel walls by following pathologic changes in heparin clearance from the blood [J. P. Esquerre et al., "Kinetics of Technetium-labeled Heparin in Thromboembolism: Preliminary Report", Int. J. Nucl. Med. and Biol., 6, 215 (1979)].
Radio-labeled fibrinogen is a positive-imaging diagnostic agent for growing thrombi but has the definite limitation in that fibrinogen is derived from human blood, and therefore, offers the possibility of contamination by a hepatitus virus. See Kenneth A. Krohn et al., "Radiopharmaceuticals for Thrombus Detection: Selection, Preparation, and Critical Evaluation", Seminars in Nuclear Medicine, 7, #3, 219 (1977). A method for labeling fibrinogen and immunoglobulins with 99m-technetium is disclosed in U.S. Pat. No. 4,057,617.
Additional radio-labeled positive-imaging diagnostic preparations are taught in the patent literature. The novel compositions described in U.S. Pat. No. 3,812,245 are radio-labeled enzymes, .sup.99m Tc-streptokinase and urokinase, stated to be useful for radiotracer localization of thromboembolisms in a vascular system. Radioactive-labeled antibodies for determining the size and location of myocardial infarcts is taught in U.S. Pat. No. 4,036,945.
In the prior art, certain chelating groups have been attached to heparin. U.S. Pat. No. 3,118,816 discloses an N-succinyl heparin which effects lipemia clearance and cholesterol lowering of the blood of warm-blooded animals, yet is substantially free of anticoagulant activity. U.S. Pat. No. 3,118,817 discloses N-(disulphobenzoyl) heparin which exhibits anticoagulant activity. Belg. Pat. No. 635,463 discloses N-(sulphobenzoyl) heparin, an anticogulant. N-benzoylheparin (L. Velluz, G. Nomine, and A. Pierdet, C. R. Acad. Sci., 247 (1958) 1521-1523), N-acetyl heparin (M. L. Wolfrom and R. Montgomery, J. Am. Chem. Soc., 72 (1950) 2859-2861) and (I. Danishefsky, H. B. Eiber, and J. J. Carr. Arch. Biochem. Biophys., 90 (1960) 114-121), N-(3,5-dimethylbenzoyl) heparin (C. Plotka and R. Jequier, Arch. Int. Pharmacodyn. Ther., 126 (1960) 140-153) and (L. Velluz, C. Platka, and G. Nomine, C. R. Acad. Sci., 247 (1958) 2203-2204), and N-acyl derivatives of heparin (S. Hirano and W. Ohashi, Carbohydr. Res., 59 (1977) 285-288) are also known. It appears novel in the art, however, to couple radioactive metal ions with chelate group-containing heparin compounds for use in locating a blood clot in a vascular system.
There remains a need in the art for imaging "aged" or formed thrombi, particularly those that are present in deep veins.
It is surprising that chelated heparin derivatives are attached to thrombi since it is known that desulfation of heparin reduces its anticoagulant activity and one would infer that it, therefore, would have little affinity for thrombi (J. Kiss, Heparin: Chemistry and Clinical Usage, Ed. V. V. Kakkar and D. P. Thomas, Academic Press, London, 1976, p. 9).